Conventionally, known as a method of forming a functional deposited film are a vacuum vapor deposition method, a high frequency (RF) plasma chemical vapor deposition method, a thermal-induced chemical vapor deposition method (hereinafter referred to as "heat CVD method"), a reactive sputtering method, an ion plating method and a light-induced chemical vapor deposition method (hereinafter referred to as "light CVD method"). These methods are selectively used depending upon the type of a functional deposited film to be formed or the desired application use of a functional deposited film formed.
For any of these film-forming methods, there still exist unsolved problems. That is, the heat CVD method is to form a functional deposited film by thermally decomposing a film-forming raw material gas with the action of heat to cause the formation of said functional deposited film on the surface of a substrate maintained at a desired temperature. However, in the case of this thermal-induced chemical vapor deposition method, there is an unavoidable disadvantage that this method can be applied only in the formation of limited kinds of functional deposited films because this method involves a high temperature heat decomposition reaction. Meanwhile, the RF plasma chemical vapor deposition method (hereinafter referred to as "RF PCVD method") is to form a functional deposited film by causing plasma discharge in a film-forming raw material gas at a relatively low temperature with the action of energy of a RF power to generate active species and chemically reacting those active species thus generated to form said functional deposited film on the surface of a substrate maintained at a desired temperature. Although the RF-PCVD method is widely used nowadays, the method is problematical due to the fact that there are a number of varied film-forming parameters which are much more complicated than the heat CVD method and those film-forming parameters are extremely difficult to be generalized. The light CVD method is to form a functional deposited film by exciting and promoting chemical reactions of one or more of film forming raw material gases with the action of light energy to form said functional deposited film on the surface of a substrate maintained at a desired temperature. The light CVD method is advantageous in the viewpoints that it is free of such a problem that the characteristics of a film to be formed by the HR-PCVD method are likely to vary because of collision of high energy particles to the substrate or/and the presence of charged particles and it makes possible to form a functional deposited film at low temperature.
However, there is a disadvantage for the light CVD method that there is a restriction in the wavelength of light which is absorbed by a film-forming raw material gas and because of this, there is a limit for the kind of a functional deposited film which can be formed by this method.
Various proposals have been made in order to solve such problems of various chemical vapor deposition methods as described above. One of such proposals provides an improved high frequency plasma chemical vapor deposition method in which hybrid excitation is utilized. The improved method will be described below.
By the way, a passivation film has been conventionally used to isolate a semiconductor integrated circuit device or a large scale semiconductor integrated circuit device from an influence of the external field in order to ensure the reliability of such semiconductor integrated circuit device, particularly of the large scale semiconductor integrated circuit device. There have been proposed a phosphor silicate film, a SiO.sub.2 film or the like as the passivation film. However, any of them does not have a sufficient blocking effect against H.sub.2 O, Na ion or the like.
Under this circumstance, the use of a Si.sub.3 N.sub.4 film as the passivation film has been discussed since it is chemically inactive and is superior to any of the phosphor silicate film and SiO.sub.2 film with respect to hardness and density and because of this, it can be expected to provide a desirable blocking effect against impurities such as H.sub.2 O, Na ions and the like. However, for the reason that it is necessary for a wafer on which said film is to be formed to be heated to an elevated temperature of 800.degree. C. or more upon formation thereof, said Si.sub.3 N.sub.4 film has not been yet put to practical use.
Recently, Mikio KOBAYAKAWA et al. have proposed a method of forming a Si.sub.3 N.sub.4 film at a temperature of about 300.degree. C. by means of PCVD technique (see, periodical journal "SHINKU" (Vacuum) vol. 31, No. 3, p. 167).
In the following, the method of forming a Si.sub.3 N.sub.4 film by the PCVD technique proposed by KOBAYAKAWA et al. will be explained with reference to the drawing (FIG. 3).
In FIG. 3, there is shown a schematic representation of a film forming apparatus by the PCVD method. Formation of a Si.sub.3 N.sub.4 film in the apparatus is carried out in the following manner. That is, a vacuum valve 4 is opened to exhaust the remaining gas in a vacuum vessel 3. Then, a gas mixture comprising SiH.sub.4 gas and NH.sub.3 gas is introduced into the vacuum vessel 3 from a gas spouting ring 22 while controlling its flow rate by a control valve 25. Thereafter, the gas pressure (inner pressure) of the vessel 3 is adjusted to 10 torr or so by mean of the valve 4, and a wafer 6 placed on a wafer holder 5 is heated to 300.degree. C. by means of a heater 24. Then, a RF (radio frequency) power (13.56 MHz) from a RF power source 1 is applied between a cathode electrode 7 and the wafer holder 5. Thereupon, an electric field is generated. Consequently, glow discharge is caused between the cathode electrode 7 and the wafer holder 5 to form plasma wherein said SiH.sub.4 gas and NH.sub. 3 gas are decomposed to generate active species which are successively chemically reacted, whereby a Si.sub.3 N.sub.4 film is formed on the surface of the water.
However, the Si.sub.3 N.sub.4 film thus formed by this PCVD method unavoidably contains about 20 atomic % of hydrogen atoms in the form of Si--H bonds or/and N--H bonds. In this respect, in the case where the Si.sub.3 N.sub.4 film is used in a MOS device, the hydrogen atoms contained in the Si.sub.3 N.sub.4 film move to the interface between a gate electrode and a silicon surface to trap hot electrons therein. This causes a change in the threshold and also a deterioration of the MOS device. There is shown an infrared absorption spectrum of a Si.sub.3 N.sub.4 film formed by the foregoing PCVD method in FIG. 5.
In order to solve the foregoing problems, utilization of a so-called hybrid excitation chemical vapor deposition method in Knudsen region (hereinafter referred to as "Knudsen region hybrid excitation CVD method") has been discussed.
The Knudsen region hybrid excitation CVD method comprises supplying a film-forming raw material gas comprising SiH.sub.4 gas and NH.sub.3 gas excited by way of a RF glow discharge toward the surface of a substrate (wafer) on which a film is to be deposited and irradiating said surface with UV-rays from an ultraviolet lamp to thereby form a deposited film on said surface activated with the irradiation of the UV-rays. With this method, the film is formed on said wafer surface as a bonded product of SiN radicals, SiH radicals and NH radicals. The film formed by this Knudsen region hybrid excitation CVD method comprises a SiN system film containing SiN component and NH component as the main constituents and SiH component in a slight amount. Upon forming the SiN film by this Knudsen method, as NH radicals absorb light having a wavelength shorter than 450 nm, they are decomposed and desorbed into a gaseous phase.
Thus, the resulting SiN system film becomes such that contains hydrogen atoms (H) in a relatively small amount of less than 10 atomic %. The Knudsen region hybrid excitation CVD method is practiced, for example, in the apparatus shown in FIG. 4.
In FIG. 6, there is shown an infrared absorption spectrum of a SiN film formed by the foregoing Knudsen region excitation CVD method by using the apparatus shown in FIG. 4.
FIG. 4 shows a schematic representation of an apparatus for forming a deposited film by the Knudsen region hybrid excitation CVD method. Referring to FIG. 4, the apparatus shown includes a high frequency power source 1, a vacuum gage 2, a reaction vessel 3 made of a conductive material, an exhaust valve 4, a wafer holder 5 for holding thereon a wafer 6 on which a deposited film is to be formed, a cathode electrode 7, a SiH.sub.4 gas spouting ring 8, a flow rate control valve 9 for SiH.sub.4 gas, a cylindrical excitation chamber 10 made of an insulating material, a flow rate control valve 11 for N.sub.2 gas, a light transmitting window 14 for transmitting light from a xenon lamp 16 therethrough, a power source 15 for the xenon lamp 16, a mirror 17 for reflecting light from the xenon lamp 17, a motor 25 for rotating the wafer holder 5 by way of a gear 26, and a power source 27 for the wafer holder rotating motor 25.
Formation of a deposited film by the Knudsen hybrid excitation CvD method using the apparatus shown in FIG. 4 is carried out, for example, in the following manner. That is, in the case where a SiN film is to be formed on a wafer 6, the reaction vessel 3 is evacuated by operating the exhaust valve 4. Then N.sub.2 gas is introduced through the excitation chamber 10 into the reaction vessel 3 while controlling its flow rate by the flow rate control valve 11. At the same time, SiH.sub.4 gas is introduced into the reaction vessel 3 while controlling its flow rate by the flow rate control valve 9. The SiH.sub.4 gas is spouted from the spouting ring 8 toward the wafer 6 placed on the wafer holder 5. The inner pressure of the reaction vessel 3 is adjusted by means of the exhaust valve 4. RF power (13.56 MHz) is applied from the RF power source 1 to the cathode electrode 7 to cause glow discharge between the cathode electrode 7 and the circumferential wall of the reaction vessel 3 being electrically grounded, so that the N.sub.2 gas is excited to generate plasmas which are successively dispersed in the entire inside space of the reaction vessel 3. The SiH.sub.4 gas spouted from the spouting ring 8 toward the surface of the wafer 6 reacts with the plasmas caused from N.sub.2 to form a deposited film on the surface of the wafer 6. In this case, ultraviolet rays from the xenon lamp 16 are irradiated onto the surface of the wafer 6. Consequently, a SiN film is formed.
With this known method, there is a problem that, since the plasmas flow into the reaction vessel 3 in one direction, the density of the plasmas in the reaction vessel becomes to be of such a distribution that shows a maximum value at a position displaced from the center of the reaction vessel 3. Actual measurement of such plasma density by the present inventor has proved that the plasma density decreases as the distance from the cathode electrode 7 increases. Specifically, when an RF power of 2.2 w/cm.sup.2 was applied to the cathode electrode 7 while the pressure of N.sub.2 gas was 10 mTorr, the plasma density was 2.times.10.sup.10 cm.sup.-3 at a position of the surface of the wafer 6 near the cathode electrode 7 but was 1.times.10.sup.9 cm.sup.-3 at the central position of the wafer 6. Now, decomposition of SiH.sub.4 gas proceeds fast where the plasma density is high, which gives a significant influence on the in-plane uniformity of the film forming rate and makes the distribution of the thickness of the film on the wafer non-uniform. In this instance, the distribution of the film forming rate was such as shown in FIG. 7(a). In order to improve this deviation, the wafer holder 5 was rotated by the motor 25 by way of the gear 26 as shown in FIG. 4 in order to make the distribution of the film forming rate uniform. In FIG. 7(b), there was shown the distribution of the film forming rate in this case.
Thus the present inventor has found the following facts on the known apparatus for practicing the hybrid excitation CVD method.
(i) Since the wafer holder is rotated in order to make the film forming rate uniform, films formed at different film forming rates overlap with each other in the thickness direction of the film, and accordingly, it is difficult to obtain a film which is uniform in quality. PA1 (ii) In order to make the film forming rate uniform by way of rotation of the wafer holder, it is desired for the film forming rate to be distributed in such a way as indicated by the broken lines shown in FIG. 7(c). However, the actual plasma density distribution becomes to present conical shapes having their apex at a position near the cathode electrode. Consequently, the film forming rate distribution becomes to present such state as indicated by the solid lines shown in FIG. 7(c) similarly to the plasma density distribution. As a result, if the wafer holder should be rotated, the film forming rate distribution becomes smaller at the central portion of the wafer than at the peripheral portion of the wafer, and accordingly, there is a limit in this case. PA1 (iii) A rotating mechanism is required to rotate the wafer holder, which complicates the constitution of the apparatus and sometimes causes troubles. PA1 (iv) Further, since the space in which plasmas are caused and the path of light for activating the surface of a substrate are not separated from each other in the conventional apparatus shown in FIG. 4, part of the plasmas caused flows in the vicinity of the light transmitting window 14. Consequently, a deposited film is often formed also on the surface of the light transmitting window 14. This causes problems that not only the light transmitting window becomes not to effectively allow transmission of light into the reaction vessel but also the film deposited thereon is peeled off and incorporated into the film to be formed. In this respect, it is necessary to periodically clean the light transmitting window 14.